Non-metallic Inclusion Control Technology and Purity Improvement Solution for National Standard Rails
What are the main types and hazards of non-metallic inclusions in national standard rails?
The non-metallic inclusions in national standard rails are mainly divided into three categories: oxides, sulfides and silicates. Oxides are mainly alumina, sulfides are mostly manganese sulfide, and silicates are aluminosilicate composite inclusions. The size and distribution of these inclusions directly affect the mechanical properties of the rail. Large-sized alumina inclusions will destroy the continuity of the rail matrix and become stress concentration points. Under the repeated action of train loads, micro-cracks are prone to occur at the interface between inclusions and the matrix, and the expansion of cracks will lead to fatigue fracture of the rail. Although sulfide inclusions have good plasticity, they will become brittle in low-temperature environments, increasing the risk of brittle fracture of the rail. Silicate inclusions are hard and brittle, which will accelerate the wear of the wheel-rail contact area, shorten the rail grinding cycle and increase the line maintenance cost.
What are the core measures to reduce non-metallic inclusions in national standard rails during the smelting process?
The core measures to reduce non-metallic inclusions in national standard rails during the smelting process are adopting secondary refining process and vacuum degassing technology. First, add slag formers such as lime in the later stage of converter smelting to form alkaline slag, which can adsorb oxide inclusions such as alumina in the molten steel. Then the molten steel is sent to the LF refining furnace, and the argon stirring is used to promote the floating of inclusions. The argon flow rate is controlled at 0.5-1.0L/min, and the stirring time is not less than 20 minutes to ensure that the inclusions are fully aggregated and discharged. The vacuum degassing technology can place the molten steel in a vacuum environment to reduce the oxygen and sulfur content in the molten steel and reduce the formation of oxides and sulfides. The vacuum degree should be controlled at ≤67Pa, and the holding time should be ≥15 minutes. In addition, aluminum wire and calcium wire feeding treatment is adopted. The reaction between calcium and aluminum generates low-melting-point calcium aluminate, avoiding the formation of high-hardness alumina inclusions. During the smelting process, it is necessary to monitor the oxygen content of the molten steel in real time and control the total oxygen content ≤20ppm to reduce the generation of inclusions from the source.
How to further optimize the inclusion distribution of national standard rails during the rolling process?
The core of optimizing the inclusion distribution of national standard rails during the rolling process is improving the inclusion morphology through plastic deformation. First, the controlled rolling and controlled cooling process is adopted, and the rolling temperature is controlled in the austenite recrystallization zone, so that the rail undergoes sufficient deformation at high temperature, and the originally irregular inclusions are elongated and broken. The deformation amount in the rough rolling stage should be ≥40%, and the large-sized inclusions are broken into small-sized inclusions through large deformation, reducing their damage to the matrix. The low-temperature rolling process is adopted in the finish rolling stage, and the temperature is controlled at 800-850℃. At this time, the plasticity of the molten steel is good, and the inclusions will be distributed in strips along the rolling direction, avoiding the formation of clustered inclusions. After rolling, the laminar cooling technology is adopted, and the cooling rate is controlled at 5-10℃/s to ensure the uniform internal structure of the rail and prevent the aggregation of inclusions due to uneven cooling. In addition, online ultrasonic testing is set up during the rolling process to monitor the distribution of inclusions inside the rail in real time, and mark and process the unqualified parts in time.
What are the detection methods and judgment standards for non-metallic inclusions in national standard rails?
The detection methods for non-metallic inclusions in national standard rails mainly include metallographic microscope method and ultrasonic flaw detection method. For the metallographic microscope method, samples need to be taken from the rail, polished, polished and corroded, and then the type, size and quantity of inclusions are observed under the microscope with a magnification of 100-500 times. During detection, three parts of the rail head, rail web and rail base should be selected, and no less than 5 fields of view should be observed for each part to count the average size and number density of inclusions. The ultrasonic flaw detection method uses a high-frequency probe with a frequency of 5-10MHz. The position and size of inclusions are judged by the reflection signal of ultrasonic waves at the interface between inclusions and the matrix. This method is non-destructive testing and suitable for batch testing in production lines. The judgment standard is based on GB/T 10561-2005. The inclusion rating of national standard rails should be ≤2 grade, among which the maximum size of alumina inclusions ≤50μm, and the number density of sulfide inclusions ≤10 pieces/mm². Rails exceeding the standard should be judged as unqualified.
What is the impact of improving the purity of national standard rails on line maintenance costs?
Improving the purity of national standard rails can significantly reduce the line maintenance cost. Rails with high purity have few internal inclusions, the probability of fatigue crack initiation is greatly reduced, the service life of the rail can be extended by more than 30%, and the frequency of rail replacement is reduced. Rails with high purity have more uniform wear at the wheel-rail contact part, and will not produce abnormal wear due to stress concentration caused by local inclusions. The grinding cycle can be extended from 6 months to 12 months, reducing the labor and equipment costs of grinding. In addition, rails with high purity have good fatigue resistance and are not prone to fracture accidents, avoiding the line outage loss caused by rail fracture, which is often much higher than the rail procurement and maintenance costs. In alpine and heavy-haul lines, high-purity rails have more obvious advantages, can adapt to harsh service environments, and further reduce the maintenance difficulty and cost of special lines.